Device and method for monitoring of absolute oxygen saturation and tissue hemoglobin concentration

ABSTRACT

A method and medical device for detecting signals that detects emitted light scattered by a volume of tissue delivered along a first pathway at a plurality of wavelengths to generate corresponding first detected light intensity output signals, detects emitted light scattered by the volume of tissue delivered along a second pathway different from the first pathway at a plurality of wavelengths to generate corresponding second detected light intensity output signals, determines whether a difference between the emitted light detected along the first pathway and the emitted light detected along the second pathway is greater than a predetermined threshold, and alters sensing by the device in response to the determining whether a difference is greater than the predetermined threshold.

RELATED PRIORITY APPLICATION

The present application claims priority and other benefits from U.S.patent application Ser. No. 12/797,744, entitled “DEVICE AND METHOD FORMONITORING OF ABSOLUTE OXYGEN SATURATION AND TISSUE HEMOGLOBINCONCENTRATION, filed Jun. 10, 2010 now allowed and to Provisional PatentApplication Ser. No. 61/185,818, filed Jun. 10, 2009, entitled “DEVICEAND METHOD FOR MONITORING OF ABSOLUTE OXYGEN SATURATION AND TOTALHEMOGLOBIN CONCENTRATION”, incorporated herein by reference in it'sentirety.

REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to the commonly-assigned related U.S.Applications: Attorney Docket Numbers P0035554.01, P0035554.02 andP0035554.03, all entitled “TISSUE OXYGENATION MONITORING IN HEARTFAILURE”, to Cinbis et al.; Attorney Docket Numbers P0035038.01 andP0035038.02, both entitled “ABSOLUTE CALIBRATED TISSUE OXYGEN SATURATIONAND TOTAL HEMOGLOBIN VOLUME FRACTION”, to Kuhn et al.; Attorney DocketNumbers P0034899.01 and P0034899.02, both entitled “SHOCK REDUCTIONUSING ABSOLUTE CALIBRATED TISSUE OXYGEN SATURATION AND TOTAL HEMOGLOBINVOLUME FRACTION” to Kuhn et al.; attorney docket number P0035742.01,entitled “ACTIVE NOISE CANCELLATION IN AN OPTICAL SENSOR SIGNAL”, toKuhn et al., and Attorney Docket Numbers P0034665.03 and P0034665.07,both entitled “DEVICE AND METHOD FOR MONITORING OF ABSOLUTE OXYGENSATURATION AND TOTAL HEMOGLOBIN CONCENTRATION” to Kuhn et al., all ofwhich are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The disclosure relates generally to medical devices and, in particular,to a medical device and associated method for monitoring oxygensaturation and total hemoglobin volume fraction.

BACKGROUND

Ambulatory monitoring of blood or tissue oxygen saturation inimplantable medical devices is generally limited to monitoring trends ofuncalibrated oxygen saturation measurements over relatively shortperiods of time. The influence of body motion, optical path length,sensor location, and the relationship of an uncalibrated oxygensaturation index to the physiological status of the tissue, e.g. toactual tissue oxygenation, can result in a broad statisticaldistribution of the response of an uncalibrated oxygen saturation indexto patient conditions. Pulse oximeters are used for bedside monitoringof a calibrated measure of hemoglobin oxygen saturation using absorbancemeasurements of red an infrared light. External, non-invasive devicesare available which use fiber optic light sources for monitoring tissueoxygen saturation based on the absorbance of near-infrared light byhemoglobin and myoglobin. However, chronic ambulatory monitoring oftissue oxygen availability in a blood-perfused tissue would be usefulfor monitoring a patient condition. A need remains for a sensor capableof monitoring calibrated tissue oxygen saturation and total hemoglobinvolume fraction that is reduced in size and power requirements for usein an implantable or wearable medical device for ambulatory patientmonitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional view of an implantable optical sensor configuredfor monitoring tissue oxygenation.

FIG. 1B is a side sectional view of an alternative embodiment of anoptical sensor for use in an implantable or wearable medical device.

FIG. 1C is a side sectional view of an alternative embodiment of anoptical sensor for ambulatory patient monitoring.

FIG. 1D is a schematic drawing of an implantable medical device (IMD)configured for both monitoring the function of and delivering therapy.

FIG. 2A is a top view of an assembly that may be used in light emittingportion of FIG. 1A-D.

FIG. 2B is a top view of an alternative embodiment of an assembly foruse in light emitting portion of FIG. 1A-D.

FIG. 2C is a side view of an alternative embodiment of a light emittingportion.

FIG. 3 is a top schematic view of an optical sensor according to anotherembodiment.

FIG. 4 is a top schematic view of a sensor according to yet anotherembodiment.

FIG. 5 is a top view of a sensor for monitoring tissue oxygenationaccording to another alternative embodiment.

FIG. 6 is a functional block diagram of a medical device associated withan optical sensor for monitoring oxygen saturation and total hemoglobin.

FIG. 7 is a flow chart of one method for operating an optical sensor formonitoring tissue oxygenation in an implantable or wearable medicaldevice.

FIG. 8 is a flow chart of a method for using an implantable or wearablemedical device including an optical sensor.

FIG. 9 is a flow chart of a method for operating an optical sensor formonitoring tissue oxygenation.

FIG. 10 is a flow chart of an alternative method for using an opticalsensor capable of measuring absolute tissue oxygen saturation formonitoring tissue oxygenation.

FIG. 11 is a flowchart of a method of monitoring tissue oxygenation in amedical device system. FIG. 11 is a flowchart of a method of monitoringtissue oxygenation in a medical device system.

FIG. 12 is a top schematic view of a sensor 600 according to anotherembodiment.

FIG. 13 is a flowchart of a method of monitoring tissue oxygenation in amedical device system.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the disclosure. For purposes ofclarity, the same reference numbers are used in the drawings to identifysimilar elements. As used herein, the term “module” refers to anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that execute one ormore software or firmware programs, a combinational logic circuit, orother suitable components that provide the described functionality.

FIG. 1A is a sectional view of an implantable optical sensor configuredfor monitoring tissue oxygenation. The term “tissue oxygenation” as usedherein refers to the availability of oxygen to a localized tissue volumeand thus refers to the availability of oxygenated hemoglobin. The term“total hemoglobin volume fraction” (HbT) refers to the concentration ofred blood cells in a measurement volume carrying hemoglobin and thusrelates to the total hemoglobin concentration as a fraction of ameasurement volume. Stated differently, the total hemoglobin volumefraction, which can be expressed as a percentage, is the volumepercentage of red blood cells carrying oxygenated and deoxygenatedhemoglobin in the measurement volume. Thus a measurement of HbT willinclude contributions from red blood cells present in any arteries,capillaries, and veins which may be present in the measurement volume.

Absolute tissue oxygen saturation (O₂Sat) is the portion (or percentage)of the total hemoglobin that is in an oxygenated state. Morespecifically, O₂Sat relates to the available hemoglobin binding sitesholding an oxygen molecule. Thus, “tissue oxygenation monitoring” asused herein refers to monitoring both O₂Sat (or an index thereof) andHbT (or an index thereof). Tissue oxygenation monitoring may involvedetermining absolute measurements of O₂Sat and HbT or determining trendsof these measurements or trends of indices of these measurements. Wheneither O₂Sat or HbT are reduced, a blood-perfused tissue can becomehypoxic.

O₂Sat measurements as described herein refer to the oxygen saturation ofthe hemoglobin present in the circulating blood in a local tissue. Assuch, any capillary, arterial and venous blood volume within the opticalpathway of the sensor will contribute to the O₂Sat measurement.Typically, the sensor will be placed over a uniform, homogenous volumeof the blood-perfused tissue so as to measure the O₂Sat in themicrocirculation of the tissue, with minimal influences from thearterial and venous blood. The measurement of O₂Sat in themicrocirculation provides a local measurement of tissue oxygensaturation whereas the oxygen saturation of arterial and venous bloodwill change systemically. The local measurement of O₂Sat is correlatedto the partial pressure of oxygen measured directly in the tissue.

Sensor 10 shown in FIG. 1A includes a light emitting portion 14 and alight detecting portion 16 configured in a sealed housing 12, which maybe hermetically sealed. Housing 12 encloses optical sensor componentsand optionally other circuitry of an associated medical device. Housing12 may be a dedicated optical sensor housing or may enclose other devicecircuitry and components in a multi-function implantable or wearablemedical device. For example, housing 12 may enclose circuitry of animplantable pacemaker or cardioverter defibrillator, an implantable orwearable physiological monitor, implantable neurostimulator, implantablefluid pump, or other implantable or wearable medical device. Housing 12may alternatively correspond to, or be mounted within, an insulative,elongated body of a medical electrical lead.

It is contemplated that sensor 10 can be implemented in association witha medical device that is fully implantable (i.e. no components extendingexternally from the patient). The wholly implantable medical device maybe contained within housing 12 and may have medical electrical leads,catheters or other components extending therefrom. In an alternativeembodiment, sensor 12 may be implemented in association with a medicaldevice that is wearable by the patient. The wearable device may becontained within housing 12 which is strapped or adhered to a patient'sskin. Housing 12 would enclose circuitry performing the functionalitydescribed herein for emitting and detecting light and computing O₂Satand HbT measurements, for use in tissue oxygenation monitoring.

The light emitting portion 14 and the light detecting portion 16 eachinclude a lens 24 and 44, respectively. Lens 24 passes light emittedfrom the light emitting portion 14 into an adjacent tissue volume 75.Tissue volume 75 encompasses a measurement volume 76 shown schematicallyin FIG. 1 and representing the volume of tissue falling within anoptical path of sensor 10 extending from light emitting portion 14 tolight detecting portion 16. Body tissue volume 75 may be any homogenousor heterogenous bodily fluid or tissue, including, but not limited to,blood, skeletal muscle, neural tissue, myocardium, skin, etc. The depthand size of measurement volume 76 within tissue volume 75 will depend onthe spacing 15 between the emitting portion 14 and the detecting portion16, among other factors such as emitting and detecting lens sizes, lightsource geometry (e.g., area of light sources and proximity to theemitting portion lens), light detector size and geometry (area of lightdetector and proximity to the detecting portion lens), cavityreflectivity within the sensor and reflectivity of external sensorsurfaces.

Light scattered by measurement volume 76 and incident on light detectingportion 16 will be passed by lens 44 into the light detecting portion16. Lens 24 and lens 44 are commonly formed from sapphire and may behermetically sealed in openings 11 and 13 formed in housing 12. Lenses24 and 44 may be sealed within openings 11 and 13, respectively, usingferrules 22 and 42, respectively. Ferrules 22 and 42 are bonded tolenses 24 and 44, e.g. using a gold braze, a polymer adhesive, or othersuitable bonding method that provides a hermetic seal between theferrule material and the lens material and is compatible with othermanufacturing processes used in fabricating sensor 10.

Housing 12 may be formed, for example, from titanium, stainless steel,ceramic, glass, or a medical grade polymer. In one embodiment, housing12 and ferrules 22 and 42 are each formed from titanium. Ferrules 22 and42 may be welded within openings formed in housing 12 to maintainhermeticity of sensor 10 and an implantable device in which sensor 10 isassembled. The optical window assembly generally disclosed in U.S. Pat.No. 5,902,326 (Lessar, et al.), hereby incorporated herein by referencein its entirety, may be implemented in embodiments of the presentdisclosure. Lenses 22 and 44 may alternatively be sealed directly tohousing 12.

Polymeric seals 26 and 46 may be formed over lenses 24 and 44 andferrules 22 and 42, respectively. Seals 26 and 46 may be formed, forexample, from silicone rubber or another material that is substantiallyoptically transparent to the wavelengths emitted by light emittingportion 14. Seals 26 and 46 protect the bond formed between the ferrules22 and 42 and the corresponding lens 24 or 44 from the corrosive effectsof bodily fluids and provide a smooth surface (which may be convex asshown) that reduces the susceptibility of sensor 10 to blood clotformation and excessive fibrotic tissue encapsulation over lenses 24 and44. Blood clot formation and fibrotic tissue encapsulation may reducelight transmission into and out of sensor 10. Though not explicitlyshown in FIG. 1A, seals 26 and 46 may extend further over ferrules 22and 42 to cover the bond between the ferrules 22 and 42 and the adjacenthousing 12.

The emitting portion 14 includes a light emitting assembly 70 coupled toferrule 20 (or directly to housing 12). Light emitting assembly 70includes light sources 32, a circuit board 28, and anoptically-insulating wall 20 surrounding light sources 32. Suitablelight sources include, without limitation, optoelectronic devices suchas light emitting diodes (LEDs), lasers such as vertical cavity surfaceemitting lasers (VCSELs), luminescent, phosphorescent or incandescentlight sources. Light source(s) 32 are mounted on a circuit board 28,which may be a printed circuit board or include wired connections orother integrated circuitry methods, to enable the necessary connectionsfor applying a drive signal, typically a current signal, to each oflight sources 32 to cause light emission. Wall 20 surrounds the lightsources 32 to prevent scattering of light within housing 12 and promotetransmission of light through lens 24 toward adjacent body tissue volume75.

Wall 20 may be formed from a rigid, light-insulating material, such as aliquid crystal polymer. Alternatively, wall 20 can be formed from otherlight-insulating materials, for example any polymer material formed as amolded component, and may be non-rigid in some applications. Wall 20 issecurely coupled to circuit board 28. Wall 20 may be coupled to circuitboard 52 by applying a coating as a hard, die coat dam holding wall 20to the board 52 as generally described in U.S. patent application Ser.No. 12/116,705, hereby incorporated herein by reference in its entirety.

Light sources 32 each emit light corresponding to spaced-apartwavelengths for use in monitoring absolute tissue oxygen saturation.Emitted light passes through lens 24 and enters body tissue volume 75.One or more light sources may be included in light emitting portion 14.The number of light sources and corresponding light emission wavelengthswill be selected according to the requirements of a particularapplication and will depend on the physiological condition or eventsthat are being.

In one embodiment four LEDs emit light at different individualwavelengths spaced apart between approximately 625 nm and approximately900 nm. While each light source is referred to herein as emitting anindividual wavelength, it is recognized that a light source, such as anLED, may emit a narrow band of wavelengths, typically centeredsubstantially on a specified wavelength.

In one embodiment, and without limitation, light sources 32 emit lightin the red to infrared spectrum as indicated by the approximatewavelengths referred to above. It is contemplated, however that otherembodiments may include light emission only in the visible spectrum oronly in the non-visible spectrum. In one embodiment light is emittedonly in the infrared spectrum, e.g. greater than approximately 720 nm,such that no visible light is emitted making light emission by thesensor less perceivable, or unperceivable, to the patient.

Light sources 32 may be mounted in a custom package 34 along a surfaceelevated from circuit board 28. In the embodiment shown, light sources32 are mounted along an upper surface 38 of a molded plastic package 34which includes a lead frame (not shown). Light sources 32 may beembodied as LEDs or other light sources and are mounted in package 34 toprotect and facilitate electrical connections between light source diesand circuit board 28. Package 34 may be formed with thermally conductivematerials to act as a heat sink for drawing heat away from the lightsources 32. Mounting light sources 32 in package 34 may also shorten thedistance between light sources 32 and lens 24.

Package 34 may include a recessed cup area 36 in which electricallyconductive die pads are positioned for receiving and coupling to thelight sources 32. Cup 36 may be filled with an optical coupling material37 to reduce the reflection of light at material boundaries, for exampleat the photonic surfaces of light sources 32 or at the inner surface oflens 24. Optical coupling material 37 filling cup 36 reduces thereflection of emitted light at component interfaces within lightemitting portion 14. More specifically, material 37 filling cup 36 has ahigh refractive index for optically coupling light sources 32 with lens24 in order to reduce the reflection of emitted light as light leavesthe photonic surfaces of light sources 32 as compared to light reflectedat the photonic surfaces when interfaced with air. Material 37 may alsobe configured to reduce reflections of emitted light at the innersurface of lens 24 as compared to reflections that would otherwise occurat a lens inner surface-to-air interface. As such, optical couplingmaterial 37 filling cup 36 may be in direct contact with the lowersurface of lens 24 and/or in direct contact with the photonic surfacesof light sources 32.

The detecting portion 16 includes a light detecting assembly 72 coupledto ferrule 42 (or directly to housing 12). Assembly 72 includes a lightdetector 52, also referred to herein as a “photodetector”, circuit board48, and an optically insulating wall 40 surrounding light detector 52.Wall 40 may have an exterior hard die coating for retaining wall 40against circuit board 48. The light detector 52 may be embodied as aphotodiode. Other components suitable for use as a light detectorinclude a photoresistor, phototransistor, photovoltaic cell,photomultiplier tube, bolometer, charge-coupled device (CCD) or an LEDreverse-biased to function as a photodiode. Light detector 52 may beprovided in a custom or commercially available package 54 including alead frame and mounted on circuit board 48 to enable appropriateelectrical connections between circuitry 50 and photodetector 52.

Wall 40 surrounds the photodetector 52 to promote light travelingthrough lens 44 to fall on photodetector 52 and minimize stray lightwithin housing 12 from reaching photodetector 52. Package 54, whenincluded, promotes reflection of light onto photodetector 52, in whichcase wall 40 primarily acts to minimize stray light within sensor 10.Wall 40 may share a common side with wall 20 in some embodiments, andmay be formed from rigid, opaque or light-insulating material, such as aliquid crystal polymer. Alternatively, wall 40 can be formed from other,light insulating materials, for example any polymer material formed as amolded component, and may be non-rigid in some applications. Wall 40 maybe attached to printed circuit board 48 using a coating applied as ahard, die coat dam holding wall 40 to the board 48.

In some embodiments, an electrically insulative material may fill openspaces between packages 34 and 54 and the respective ferrules 22, 42 andhousing 12 to prevent electrical coupling between components mounted inpackages 34 and 54 and ferrules 22, 42 and housing 12. When housing 12serves as an active electrode, for example, when housing 12 forms thehousing of an implantable cardioverter defibrillator, a minimum spacingmay be required between the housing 12 and the light sources 32 andlight detector 52.

While separate circuit boards 28 and 48 are shown, it is contemplatedthat a single circuit board may be provided with both the light emittingassembly 70 and the light detecting assembly 72 mounted thereon. Thesingle circuit board or the separate circuit boards 28 and 48 mayadditionally include circuitry corresponding to other functions of animplantable medical device in which sensor 10 is incorporated.

Integrated circuitry 30 included on circuit board 28 is electricallycoupled to light sources 32 to deliver drive signals to activate lightsources 32 to emit light. Integrated circuitry 50 included on circuitboard 48 is coupled to photodetector 52 to receive the electrical signalemitted by photodetector 52 in response to scattered light incident onphotodetector 52. Circuitry 50 provides a signal to processing circuitrywhich may be housed within housing 12 or otherwise included in anassociated medical device. Processing circuitry, as will be describedherein, may be included on circuit board 48 or implemented separatelyand is configured to compute O₂Sat and Hbt using the photodetectoroutput signal and perform a monitoring algorithm, e.g. for monitoringoxygenation of tissue volume 75. Integrated circuitry 50 may include ananalog-to-digital converter and memory for digitizing the analog outputsignal from photodetector 52, providing the digitized signal to theprocessing circuitry, storing measurement results for future retrievalas well as storing calibration coefficients. In other embodiments, onlyraw signals are collected and stored, then transmitted to a processor,which may be included in an implantable or wearable device or located inan external device such as a programmer or computer, where the raw datais processed to determine calibrated O₂Sat and Hbt values.

In some embodiments, sensor 10 additionally includes a referencephotodetector 38 in light emitting portion 14. The light entering tissuevolume 75 from emitting portion 14 may change over time during chronicuse of sensor 10 due, for example, to drift in the photonic output oflight sources 32 and/or changes in the optical properties of seal 26.Relative changes in oxygen saturation over relatively short periods oftime, for example seconds, minutes, hours or perhaps even days, will notbe significantly affected by drift of the light source output or foulingof the seal 26. Changes in the intensity of the emitted light due tosuch causes will be gradual and may only occur over a period of monthsor years. Such changes will thus have a substantially equal effect onboth an initial measurement and a subsequent measurement used to computea relative oxygen saturation change over relatively shorter durations oftime such as seconds, minutes, hours, or days. In contrast, an absolute,calibrated measurement of tissue oxygen saturation may become erroneousover time if the relative intensity of the emitted light for each lightsource changes over time without compensation.

Control circuitry 30 may include circuitry for receiving an outputsignal from reference photodetector 38 for detecting changes in theintensity of the light emitted by emitting portion 14 over time for usein computing or adjusting O₂Sat and HbT and/or for use in controllinglight emission as will be further described below. In some embodiments,the computation of absolute tissue oxygen saturation using an outputsignal from photodetector 52 also requires a measurement of the lightintensity emitted by emitting portion 14 (which can be measured byreference photodetector 38) in order to compute the attenuation of lightat predetermined wavelengths. In other embodiments, adjustments ofcalculated tissue O₂Sat are made based on changes in the intensity oflight emitted by emitting portion 14 since an initial emitted lightintensity measurement was made.

When multiple light sources 32 are included for emitting light atseparate spaced apart wavelengths, light emitting portion 14 may includea light diffusing material to diffuse the light emitted by each lightsource before it enters tissue 75. Diffusion of separately emittedwavelengths before entering tissue 75 will promote similar opticalpathways of the separate wavelengths through tissue 75. A lightdiffusing material may be an epoxy filled with glass beads to promotelight scattering. A light diffusing material may be included in thematerial filling cup 36, coated over the top surface of package 34, orcoated on an inner or outer surface of lens 24.

FIG. 1B is a side sectional view of an alternative embodiment of anoptical sensor 10′ for use in an implantable or wearable medical device.Sensor 10′ includes an emitting portion 14′ and a detecting portion 16′housed in a hermetically sealed housing 12′. Housing 12′ includesopenings in which lenses 24′ and 44′ are sealed, for example using epoxyor other sealing adhesive. A light emitting package 34′, including oneor more light sources 32′, is mounted directly to an inner surface oflens 24′. A light detecting package 54′, including a light detector 52′,is mounted directly to an inner surface of lens 44′. An epoxy or otheradhesive that is optically transparent at the wavelengths of interestmay be used to mount packages 34′ and 54′ directly to respective lenses24′ and 44′.

Conductors 61, which may be laser ribbon bonds, wires, flexiblecircuits, or the like, electrically couple the light emitting package34′ and the light detecting package 54′ to circuitry 62 mounted oncircuit board 60. As described previously, circuitry 62 includescircuitry for providing drive signals to light sources 32′ in lightemitting portion 14′ and for receiving the output signal from lightdetecting portion 16′.

A black polymer 59 is dispensed over the back side of packages 34′ and54′ to prevent stray light within housing 12′, provide opticalinsulation of the light detector 52′, and add mechanical stability tothe light emitting and light detecting portions 14′ and 16′.

FIG. 1C is a side sectional view of an alternative embodiment of anoptical sensor 63 for ambulatory patient monitoring. One or more lightemitting portions 67 and one or more light detecting portions 68 areassembled along opposing inner surfaces of a generally “C”-shaped orclamshell style cuff 65. Cuff 65 may extend from a distal end of anelongated lead 66. Conductors 64 may extend from each of the lightemitting portions 67 and light detecting portions 68 to be electricallycoupled to a lead connector at a proximal lead end for electricalconnection to an associated medical device.

Alternatively, cuff 65 may be a leadless device. Cuff 65 may housefunctional circuitry (as generally described below) needed for sensor 63to operate a wireless device.

Cuff 65 may be provided as a flexible device, for example fabricatedfrom a flexible polyurethane or silicone material. Alternatively cuff 65may be fabricated from a rigid material that rigidly maintains opposingalignment of an emitting portion 67 with a detecting portion 68. Cuff 65may be formed as a single, continuous piece as shown in FIG. 1Cincluding one or more emitting and detecting portions 67 and 68 infacing opposition along a housing. Alternatively, cuff 65 may includetwo separate, sealed housings, one for housing the emitting portions 67and another separate housing for the detecting portions 68. Cuff 65 maythen include a rigid or flexible C-shaped connector extending betweenthe two separate housings to form the generally C-shaped cuff 65.

Cuff 65 may be provided in varying sizes to allow sensor 63 to be placedaround a body tissue 75′ of interest, which may be, for example, a bloodvessel, a skeletal muscle, a digit, the thenar muscle in the palm of thehand, or a rib and adjacent intercostal muscle tissue. By positioningemitting and detecting portions 67 and 68 spaced apart and in facingopposition to each other, light emitted by an emitting portion 67 andtransmitted through tissue 75′ is received by light detecting portion68. The measurement volume will be defined by an optical pathway that isapproximately linear through the tissue 75′ that is “sandwiched” betweenthe emitting and detecting portions 67 and 68. Shifts in the opposingalignment of an emitting portion 67 and a detecting portion 68 may betolerated since absolute O₂Sat measurements described herein arerelatively independent of the volume of tissue in the optical pathway ofthe sensor (when the tissue is a uniform, homogenous tissue). Ifshifting does occur, resulting in measurements through an undesiredoptical pathway in non-uniform or non-homogenous tissue or produceserroneous measurements of the tissue oxygenation, the inclusion ofmultiple emitting and detecting portions 67 and 68 allows an optimalemitting portion and detecting portion pair to be selected for achievingan optimal output signal.

FIG. 1D is a schematic drawing of an implantable medical device (IMD) 80configured for both monitoring the function of and delivering therapy toheart H. In FIG. 1, heart H is shown in a partially cutaway viewillustrating right atrium RA, right ventricle RV, left ventricle LV, andcoronary sinus CS.

IMD 80 is shown embodied as an ICD that includes a pulse generator fordelivering electrical stimulation to heart H for use in cardiac pacingtherapies, cardioversion and/or defibrillation. Another example of animplantable medical device in which methods described herein may bepracticed would be a subcutaneous cardioverter/defibrillator havingelectrodes implanted subcutaneously rather than transvenously asdescribed herein.

IMD 80 includes hermetically-sealed housing 81, connector block assembly82, right atrial (RA) lead 83, right ventricular (RV) lead 84, leftventricular (LV) lead 85, and optical sensor lead 86. IMD 80 furtherincludes circuitry and a power source, which are located within housing81, for controlling the operation of IMD 80. The circuitry communicateswith leads 83-86 through electrical connectors within connector blockassembly 82. A can electrode is formed on or is a part of the outersurface of housing 81, and may act as an electrode in a unipolarcombination with one or more of the electrodes carried by leads 83-85.

Leads 83-85 extend from connector block assembly 82 to right atrium RA,right ventricle RV, and coronary sinus CS adjacent left ventricle LV,respectively, of heart H. Leads 83-85 each carry one or more electrodesfor sensing EGM signals attendant to the depolarization andrepolarization of heart H, for providing pacing pulses for causingdepolarization of cardiac tissue in the vicinity of the distal endsthereof, and for providing cardioversion/defibrillation shocks. Whenprovided, a shock is typically delivered between a combination ofelectrodes carried on RA and RV leads 83 and 84 and the can electrode.

IMD 80 may include an optical sensor 88 along the housing 81 foremitting light into a tissue volume adjacent IMD 80 and detecting lightscattered by the tissue volume for measuring light attenuation by thetissue. The measured light attenuation is used to compute tissueoxygenation measurements as will be described herein.

Alternatively or additionally, an optical sensor 87 may be carried by alead 86 extending from IMD 80. Lead 86 extends from connector blockassembly 82 to optical sensor 87, which is extravascularly-implanted,typically subcutaneously or submuscularly, at a desired tissue site. Inother embodiments, sensor 87 may be carried by a lead and placedtransvenously or transarterially in the blood stream itself. Alead-based sensor may be positioned to transmit light outward throughthe wall of a vessel to monitor perfusion in adjacent tissue.

Sensor 87 may alternatively be embodied as a wireless sensor, implantedremotely from IMD 80 or worn externally by the patient. Sensor 87provided as a wireless sensor includes telemetry circuitry for wirelesstelemetric communication with IMD 80.

FIG. 2A is a top view of light source package 34 that may be used in thelight emitting portions of the sensors shown in FIGS. 1A-1D. Package 34includes a plastic molded substrate 35 including a lead frame havingmultiple exposed electrodes 93. Package 34 includes recessed cup portion36 having die pads (not explicitly shown in FIG. 2A) to which lightsources 32 a-d (also referred to collectively as 32) and photodetector38 are mounted and electrically coupled. Package 34 facilitatesconnection of light sources 32 to drive circuitry located on a circuitboard via respective leads of the lead frame extending within substrate35 to individual exposed electrodes 93. Similarly, referencephotodetector 38 is electrically coupled via a respective electrode 93to output circuitry located on a circuit board for providing a referencelight signal corresponding to the intensity of light emission by lightsources 32.

Package 34 optionally includes a heat sink material that absorbs heatproduced by light sources 32. Package 34 may be mounted on a circuitboard 28 as shown in FIG. 1 such that electrodes 93 are electricallycoupled to control circuitry 30 (FIG. 1) included on a circuit board 28,e.g. by printed or wired connections. Alternatively, package 34 ismounted directly to a lens inner surface as shown in FIG. 1B and iselectrically coupled by conductors extending to a circuit board.

In one embodiment, the four light sources 32 a-d are selected to emitlight in the red to infrared spectrum. In one embodiment one lightsource 32 a is selected to emit red light at a wavelength ofapproximately 660 nm to approximately 680 nm. Another light source 32 dis selected to emit infrared light at a wavelength of approximately 800to approximately 890 nm. The remaining two light sources 32 b and 32 care selected to emit red or infrared light at wavelengths that areintermediate the wavelengths of light sources 32 a and 32 d. Forexample, light source 32 b is selected to emit light at a wavelength ofapproximately 720 nm, and the other light source 32 c is selected toemit light at a wavelength of approximately 760 nm. In one embodiment,40 nm spacing of the wavelengths is selected, e.g., 680 nm, 720 nm, 760nm, and 800 nm, though other wavelength spacings could be used.Wavelength spacings may be selected to maximize the sensitivity ofsensor output signals to O₂Sat and HbT. Wavelengths may be equally orunequally spaced.

Reference photodetector 38 is provided to measure the intensity of lightemitted by light emitting portion 14. In one embodiment, photodetector38 is exposed to lens 24 as shown in FIG. 1 such that light detected byphotodetector 38 includes light emitted by light sources 32 a-d andlight remitted (i.e., scattered) by adjacent tissue and incident on theemitting portion 14 and passing back through lens 24. In thisconfiguration, an intensity input measurement by reference photodetector38 will include effects of drift or other changes in light source outputas well as effects of fouling of seal 26 or other changes in the opticalproperties of sensor 10 which may affect the intensity of light emittedfrom emitting portion 14 and entering tissue volume 75.

FIG. 2B is a top view of an alternative embodiment of a light sourcepackage 34″ for use in light emitting portion 14 of FIG. 1A. Identicalcomponents in assembly 34″ are identified by the same reference numeralsas in package 34 of FIG. 2A. In package 34″, an optical shield 96 ispositioned to minimize the remitted light that passes through lens 24into emitting portion 14 from directly reaching photodetector 38.Optical shield 96 may be formed as a coating on an inner or outersurface of lens 24 (shown in FIG. 1), or over the top surface of package34″. Optical shield 96 may alternatively be formed as a separatecomponent that is positioned between lens 24 and package 34″, overphotodetector 38. An inner surface 97 of shield 96 that facesphotodetector 38 may be provided as a substantially flat surface 97 oras curved surface to direct light from light sources 32 ontophotodetector 38.

Shield 96 may be reflective or absorptive and may have differentproperties on an outer surface 95 facing toward tissue 75 than on itsinner surface 97 facing toward photodetector 38. For example, shield 96may be diffuse reflective or absorptive on the inner surface 97 facingphotodetector 38, but may be reflective on the outer surface 95 facingtissue 75 so as to promote propagation of secondary reflections intotissue 75.

In this embodiment, reference photodetector 38 provides an output signalcorresponding to the light intensity emitted by light sources 32 a-dthat is more strongly dependent on the intensity of light emitted bylight sources 32 and relatively less dependent on remitted lightentering light emitting portion 14. In this way, the output signal ofreference photodetector 38 is less influenced by changes in emittedlight intensity that occur as the result of fouling of seal 26 or othermaterial factors that may attenuate the light emitted by light sources32 a-d before it enters an adjacent tissue. Reference photodetector 38may be used to monitor the performance of light sources 32 and provide afeedback signal for controlling the drive signals applied to lightsources 32. The reference signal may be used in computing absoluteoxygen saturation, to control light source drive signals, and/or adjustcomputed oxygen saturation measurements.

FIG. 2C is a side view of an alternative embodiment of a light emittingportion 114. In this embodiment, the ferrule 122 and opening in housing112 are sized such that lens 124 and seal 126 are positioned over lightsources 132. Reference photodetector 138 is positioned within housing112 and insulating wall 120 away from lens 124 such that photodetector138 is not directly beneath any portion of lens 124. Referencephotodetector 138 is positioned directly beneath a portion of housing112 and/or ferrule 122 such that photodetector 138 is not directlyexposed to light scattered back into light emitting portion 114 throughlens 124. It is recognized that some portion of remitted light scatteredor reflected within wall 120 may fall upon photodetector 138. However,photodetector 138 is expected to provide an output signal that is morestrongly dependent on the intensity of light emitted by light sources132 and relatively less dependent on remitted light entering lightemitting portion 114 through lens 124.

By selecting a distance 136 between photodetector 138 and housing 112,and/or selecting the optical properties of the surfaces within emittingportion 114, for example along inner surfaces of housing 112 and/or wall120, the proportion of emitted light reaching photodetector 138 fromlight sources 132 and the proportion of remitted light reachingphotodetector 138 from adjacent tissue through lens 124 may beselectively controlled to improve the sensitivity of the photodetectoroutput signal to light source emission. As described in conjunction withFIG. 2B, this reference photodetector 138 provides a reference signalthat may be used to monitor and control light source 132 performance.The reference signal may be used in computing absolute oxygensaturation, to control light source drive signals, and/or adjustcomputed oxygen saturation measurements.

FIG. 3 is a top schematic view of an optical sensor 75 according toanother embodiment. A photodetector 76 is positioned beneath lens 79 ina light detecting portion. Lens 79 includes four different filters 78 athrough 78 d. Filters 78 a-d may be embodied as four different coatingsapplied to lens 44 each selected to pass a desired wavelength ofremitted light. In other embodiments, filters 78 a-d may include athermally or electrically-actuated filter mechanism, such as a rotatingwheel, or a tunable filter such as an etalon. Sensor 75 may include awhite light source 77 in the emitting portion, or multiple lightsources, such as LEDs, emitting separate wavelengths concurrently.Remitted light passing through lens 79 will include a spectrum of lightwavelengths encompassing at least four wavelengths for which attenuationmeasurements will be made.

Filters 78 a-d enable photodetector 76 to produce an output signalincluding components of the remitted light corresponding to each of thefour wavelengths filtered by filters 78 a-d. In alternative embodiments,multiple photodetectors may be provided in the light detecting portion.Each photodetector may receive scattered light via a respective lightfilter and produce an output signal corresponding to a particularwavelength or narrow wavelength band. Alternatively, each photodetectormay be provided as a reverse biased LED to function as a narrow-bandphotodiode as generally described in U.S. patent application Ser. No.11/955,025, hereby incorporated herein by reference in its entirety.

FIG. 4 is a top schematic view of a sensor 100 according to anotherembodiment. Sensor 100 includes two modular assemblies 102 and 102′ eachmanufactured to have identical light emitting and light detectingcomponents. Specifically, assemblies 102 and 102′ each include lightsources 106 and 106′ respectively and light detectors 108 and 108′,respectively. The functionality of each assembly 102 and 102′ isselectable such that one assembly 102 or 102′ is selected to operate asan emitting portion and the other assembly 102 or 102′ is selected tooperate as a detecting portion. This functional selection may be made atthe time of manufacture and not alterable thereafter. Alternatively,this selection may be dynamic under the control of control circuitryincluded in the associated medical device. Functional selection ofassemblies 102 and 102′ may be based on user input or in response tofeedback or self-diagnostic measurements made by sensor 100.

By arranging the light sources 106 and 106′ and the light detectors 108and 108′ in a particular spatial manner with respect to one another, twodifferent optical pathways 110 and 112, and thus two differentmeasurement volumes, may be realized depending on the selection of thefunctionality of each assembly 102 and 102′. In the exampleconfiguration shown, the assemblies 102 and 102′ are positioned next toeach other such that the photodetector 108 of portion 102 is nearest thelight sources 106′ of assembly 102′. Likewise, the light sources 106 ofassembly 102 are farthest from the photodetector 108′ of assembly 102′.If assembly 102 is selected as the detecting portion and assembly 102′is selected as the emitting portion, the emitting-to-detecting spacing104 is relatively shorter than a spacing 105 that would result iffunctional selection of assemblies 102 and 102′ were reversed (i.e.,assembly 102 selected as emitting and assembly 102′ selected asdetecting). The longer emitting-to-detecting spacing 105 will result ina relatively longer (and deeper) optical pathway 112 (shownschematically) when assembly 102 is emitting light than the opticalpathway 110 (shown schematically) that results when assembly 102′ isemitting light.

As such, through proper orientation of assemblies 102 and 102′ and theemitting and detecting components therein, different optical pathwaysmay be selected through the selectable functionality of the assemblies102 and 102′. While a single configuration is shown in FIG. 4, it isrecognized that numerous arrangements of two or more assemblies 102 and102′ and the emitting and detecting components therein may be conceivedto allow multiple light emitting and light detecting combinations to beselected, each combination corresponding to a different optical pathwaythrough a tissue volume. The different optical pathways may differ inlocation along the tissue and/or may differ in length (depth) within thetissue.

When one modular assembly, e.g. assembly 102, is selected as theemitting portion, the photodetector 108 in the emitting portion assembly102 may operate to provide a reference signal corresponding to emittedlight intensity. Alternatively, the photodetector 108 in assembly 102selected to operate as an emitting portion may be functionally disabledduring light emission. The light sources 106′ in the other module 102′selected to operate as a light detecting portion in this example will beinactive during light detection by photodetector 108′.

In other embodiments, assembly 102 selected as the emitting portion mayinclude light detection by photodetector 108 to yield measurements at avery shallow tissue depth. A partial mask between the photodetector 108and light sources 106 may be used to prevent photodetector saturationbut allow some direct shunting of emitted light to photodetector 108 todetect drift of the photonic output of light sources 106. Shallowmeasurements may be useful to indicate a thickness of a fibrousencapsulation of the sensor. A photodetector 108 included in an assembly102 may serve the dual purpose of light detector for light sources 106′in another assembly 102′ and providing a reference measurement ofemitted light intensity for assembly 102.

By including modular assemblies 102 and 102′ that can function as eitheremitting or detecting portions, the functionality of each assembly canbe switched if a light source or a light detector in one assembly fails.The functionality may also be switched during tissue oxygenationmonitoring to obtain measurements at different tissue depths andvolumes. Measurements at different tissue depths can provide a measureof tissue uniformity, as will be further described herein. Briefly, anO₂Sat measurement using optical pathway 110 may be compared to an O₂Satmeasurement using optical pathway 112. Similarity between the twomeasurements indicates tissue uniformity since the O₂Sat measurementscomputed as generally described herein are independent of themeasurement volume defined by the optical pathway of the sensor in auniform, homogenous tissue. A significant difference between the twomeasurements indicates tissue heterogeneity or non-uniformity(oxygenation gradient through the tissue). Based on measurementcomparisons, one optical pathway 110 or 112 may be more desirable foruse in monitoring tissue oxygenation. For example, one pathway 112 maypenetrate through an adjacent tissue volume and reflect off of bone,other device components, or enter other undesired tissue. Comparisons ofoxygenation measurements obtained using different optical pathways maybe used in selecting the functionality of the assemblies 102 and 102′.

Measurements at different tissue depths or along different pathways mayalso provide measurement redundancy to promote confidence in O₂Sat andHbT measurements. Manufacturing of identical modular assemblies cansimplify manufacturing processes and potentially reduce costs.

In FIG. 4, the light sources 106 are arranged linearly such that eachindividual light source is approximately the same distance 105 fromphotodetector 108. Differential spacing between each light source 106and photodetector 108 may have significant effects on O₂Sat and HbTmeasurements due to differing optical path length for each light sourceand differing tissue properties within each optical path of the separatewavelengths when the adjacent tissue volume is non-uniform orheterogeneous. The magnitude of the effects of different optical pathlength and tissue heterogeneity or oxygenation gradients may beminimized by minimizing the differential spacing between separate lightsources 106 and photodetector 108. Spatial arrangements of light sources106 relative to photodetector 108 that remove or minimize anydifferential spacing are desirable. When the overall distance 105between light sources 106 and photodetector 108 is relatively largecompared to spacing between individual light sources 106 and theresulting differential spacing to photodetector 108, the effects ofdifferential spacing of light sources 106 to photodetector 108 isreduced. For example, when distance 105 is approximately 20 timesgreater (or more) than the spacing between light sources 106,measurement error due to differences in optical path length betweenindividual light sources 106 and photodetector 108 may be insignificant.Likewise, if the overall distance 105 is large compared to thecharacteristic size of tissue structures that introduce heterogeneity inthe tissue, the effects of different optical pathways (having differentoptical properties) through the tissue for each individual light sourcewill be reduced. As such, to minimize measurement error, it is desirableto place the optical sensor over substantially homogeneous tissue,maximize the distance 105 between light sources 106 and photodetector108, and minimize the differential distances between individual lightsources and photodetector 108.

Furthermore, assembly 102 (and 102′) may include a light diffusingmaterial as described previously to diffuse separately emitted lightwavelengths and thereby reduce the effects of differential spacingbetween individual light sources 106 and photodetector 108. The size ofthe window in the housing and the associated lens may also be minimizedto reduce any differences in optical pathways between individual lightsources 106 and photodetector 108. For example, if the light sources aregrouped in a square of four as shown in FIGS. 2A and 2B, an opening inthe housing and the associated lens over the grouped light sources maybe centered on the square grouping with a diameter no greater than thesquare to create a point of light emission that is similar for theseparate light wavelengths.

An optimal distance 105, which is also referred to herein as the“emitting-to-detecting spacing”, for a particular sensing applicationwill be dependent on a number of factors. For implantable or wearabledevices, it is generally desirable to minimize the distance 105 toreduce the overall device size and power requirements. However, reducedemitting-to-detecting spacing can increase measurement error asdescribed above and will limit the depth of the optical pathway withinthe adjacent tissue volume. As such, multiple design considerations willbe taken into account when selecting the emitting-to-detecting spacing105 for a particular monitoring application. As an example anemitting-to-detecting spacing 105 may be anywhere between approximately5 mm and approximately 25 mm, however embodiments described herein arenot limited to any particular emitting-to-detecting spacing.

FIG. 5 is a top view of a sensor for monitoring tissue oxygenationaccording to an alternative embodiment. Sensor 140 includes lightemitting portion 142 and light detecting portion 144, which maycorrespond to any of the light emitting and light detecting portionsdescribed above. Sensor 140 additionally includes a temperature sensor146. Temperature sensor 146 may utilize a thermistor, a thermocouple, aP-N diode, a junction of a bipolar junction transistor (BJT) device, orother temperature sensitive device such as the absolute temperaturesensing circuit generally described in U.S. Pat. No. 6,682,135 (Davis,et al.), hereby incorporated herein by reference in its entirety.Temperature sensor 146 is provided to account for changes in tissueoxygenation caused by changes in temperature. Temperature data may beused in discriminating between possible causes of tissue oxygenationchanges. Temperature monitoring can be used to remove, adjust orotherwise correct for the influence of temperature on tissue oxygenationmeasurements.

In one embodiment, sensor 140 includes a heating element 148. Whileheating element 148 is shown schematically in FIG. 5 as being confinedto a specific area of sensor housing 141, it is recognized that heatingelement 148 may be incorporated over a larger area of sensor 140 and/oran associated medical device to allow uniform tissue heating and may beimplemented using multiple discrete elements spaced apart to promoteeven tissue heating across a desired surface area of the tissue. Heatingelement 148 may be a resistor or other element that can be controlled toproduce heat without causing undue overheating of the adjacent tissue orharming electronics within sensor 140 or an associated medical device.Heating element 148 may be mounted on an outer or inner surface of thehousing 141 of sensor 140 and may be backed by a thermally insulativematerial to prevent heating of other components within sensor 140.

Controlled heating of the adjacent tissue may be used in a number ofways during tissue oxygenation monitoring. In one embodiment, heatingelement 148 may be used to heat the adjacent tissue volume to apredetermined temperature whenever tissue oxygenation measurements areacquired to provide temperature-controlled measurements thereby reducingthe variability of oxygenation measurements that may be caused bytemperature variation. Temperature sensor 146 may be used to verify whenthe adjacent tissue temperature has reached a desired temperature toenable oxygenation measurements and prevent tissue overheating.

In other embodiments, tissue heating may be applied to causevasodilation which will raise the arterial contribution to the localtissue oxygenation measurements. Depending on the placement of sensor140, tissue oxygenation measurements will generally include acontribution from capillary blood volume and may include a contributionfrom venous blood volume and arterial blood volume. For monitoringoxygenation in a tissue such as skeletal muscle, it is generallydesirable to avoid a large contribution from arterial or venous bloodvolume. The oxygen saturation and total hemoglobin in the capillariesare expected to best reflect the availability of oxygen to the localtissue. However, arterial oxygen saturation monitoring may be desirablein some applications to provide a systemic measurement of O₂Sat. Byheating the tissue locally to cause arterial dilation, a measuredabsolute O₂Sat may approach arterial blood oxygen saturation andtherefore be used to monitor arterial oxygen, even when sensor 75 is notplaced directly against an artery. The arterial oxygen saturationmeasurement (at an increased temperature to cause vasodilation) andlocal tissue oxygenation measurements (at an intrinsic tissuetemperature) may be used alone or in combination to detect anddiscriminate patient conditions.

In some embodiments, multiple measurements of tissue oxygenation may beobtained as the local tissue temperature is adjusted incrementally todifferent levels under the control of heating element 148 andtemperature sensor 146. Such measurements may allow for observation ofnormal or abnormal tissue oxygenation response to temperature change.

It is recognized that the incorporation of heating element 148 is notlimited to embodiments including a temperature sensor 146 and viceversa. A heating element 148 may be included that reliably produces acontrolled amount of heat in a repeatable manner such that temperaturesensing is not required for controlling and monitoring tissue heating.Likewise, temperature monitoring by sensor 146 may be used withoutartificially altering the intrinsic tissue temperature using heatingelement 148.

While not explicitly shown in FIG. 5, heating element 148 andtemperature sensor 146 will be electrically coupled to control andoutput circuitry included in housing 141 to allow control of heatingelement 148 and to receive and process temperature sensor signals.

In some embodiments, optical sensor 140 includes electrodes 143 and 145along sensor housing 141 to provide electrical stimulation of excitabletissue adjacent to sensor 140. Electrical stimulation of adjacent tissuewill increase the tissue metabolic demand for oxygen. The availabilityof oxygen to the local tissue before, during and/or after electricalstimulation may be monitored by measuring oxygen saturation and totalhemoglobin volume fraction using optical sensor 140. Changes in theavailability of oxygen with changes in metabolic demand may beevaluated. Electrodes 143 and 145 are electrically coupled to pulsegeneration circuitry (not shown) within housing 141 and the applicationof stimulation pulses would be controlled by sensor control circuitry.

FIG. 6 is a functional block diagram of an implantable medical device150 associated with an optical sensor for monitoring O₂Sat and HbT.Device 150 includes an optical sensor 180, which may correspond tosensor 10 of FIG. 1 or any of the other sensor embodiments describedherein. Device 150 further includes sensor input circuitry 162, sensoroutput circuitry 166, and optional reference signal output circuitry 164if a reference photodetector is included in light emitting portion 182of the optical sensor 180. Sensor input circuitry 162, which maycorrespond to circuit 30 of FIG. 1A, is coupled to a light emittingportion 182 of sensor 180. Sensor input circuitry 162 provides inputsignals to the optical sensor 180. In particular, sensor input circuitry162 provides the drive signals applied to light sources in lightemitting portion 182 to cause controlled light emission.

Sensor input circuitry 162 is controlled by sensor control module 168which coordinates the time, duration, and frequency of drive signalsproduced by sensor input circuitry 162. A period of no light emissionmay be included to allow ambient light measurement. In one embodiment,the drive signals are applied sequentially to cause sequential (i.e.,non-simultaneous) light emission at separate spaced-apart wavelengths byindividual light sources. In this way, the detecting portion 184 willreceive scattered light at an individual wavelength during any giveninterval of active light emission by sensor 180. It is recognized thatreferring to an “individual” wavelength can include a narrow bandwidthof wavelengths approximately centered on a specified wavelength emittedby the light source. The sequential emission of light wavelengths,referred to herein as “time multiplexing”, allows multiple, separatelight signals to be acquired and processed. A single O₂Sat and HbTmeasurement will require some minimum interval of time to perform whichcorresponds to the cumulative time durations of each of the separatelyemitted wavelength signals.

In alternative embodiments, the sensor input circuitry 162 is controlledby sensor control module 168 to deliver drive signals simultaneously toeach of the light sources. Simultaneous emission of spaced-apartwavelengths by multiple light sources may be used when wavelengthfiltering is applied by the detecting portion 184. Alternatively,simultaneous emission of multiple, spaced-apart wavelengths may becontrolled to emit each of the wavelengths at separate, uniquefrequencies. Each light source will emit light having a signaturefrequency fluctuation and is referred to herein as “frequencymultiplexing”. The detecting portion 184 will receive scattered light atall of the wavelengths simultaneously with each wavelength modulated toa signature frequency. The photodetector signal is then demodulated toobtain the individual wavelength signals.

Simultaneous emission with filtering or frequency multiplexing methodsof controlling light emission allows simultaneous light emission anddetection of all wavelengths of interest. Changes in light attenuationdue to oxygen and hemoglobin changes in the tissue volume can bemeasured simultaneously for all of the wavelengths rather than atdiscrete time intervals for separately emitted wavelengths. This allowsfor a more instantaneous measurement of O₂Sat and HbT as compared to thetime-multiplexed method of controlling light emission.

The different wavelengths may be modulated at frequencies that are muchgreater than the frequency of ambient light changes. Demodulation of thedetected light signal will reduce or eliminate effects of ambient lightsince the low frequency components of the detected light signalcorresponding to ambient light changes will be substantially removedfrom the demodulated photodetector output signal.

Sensor output circuitry 166 receives the photodetector signal fromdetecting portion 184 and demodulates and digitizes the signal toprovide a digital signal to monitoring module 170. Monitoring module 170uses the optical signal to compute an absolute O₂Sat and a HbT. Theabsolute O₂Sat is provided to a processor 154 (or other controlcircuitry) for detection of a physiological condition, determining aneed for delivering or adjusting a therapy, and/or monitoringeffectiveness of a delivered therapy.

It is recognized that IMD 150 may include other sensors for sensingphysiological signals such as intracardiac EGM or ECG signals, bloodpressure, patient activity, patient posture, or the like. Such sensorsignals may be used in combination with the monitored absolute tissueoxygen saturation and total hemoglobin volume fraction for determiningwhen a therapy is needed and delivered by therapy delivery module 156.Therapy delivery module 156 may include electrical pulse generationcapabilities for delivering cardiac pacing, cardioversion ordefibrillation or nerve stimulation. Therapy delivery module 156 mayadditionally or alternatively include a fluid delivery pump fordelivering a pharmaceutical or biological fluid to the patient.

Data acquired by processor 154 relating to O₂Sat and HbT measurementsmay be stored in memory 152 and/or transferred to a medical deviceprogrammer, home monitor, computer, or other external or bedside medicaldevice via telemetry module 158 for review by a clinician. Processor 154transmits data to and from memory 152, therapy delivery module 156, andtelemetry module 158 via data/address bus 160.

In embodiments including a reference photodetector in the emittingportion 182, reference signal output circuitry 164 provides an outputsignal to sensor control 168 and/or to monitoring module 170. In oneembodiment, the reference signal output circuitry provides an emittedlight intensity feedback signal to sensor control 168 in a feedbackcontrol loop to maintain emitted light at a desired intensity. Eachlight source drive signal is automatically adjusted to maintain theemitted light at a desired intensity (or within a desired range). Inthis way, the emitted light spectra is reliably maintained over timepromoting the accuracy of O₂Sat and HbT measurements computed usinginitial calibration constants and assuming stable light emissionintensity. Accordingly, sensor control 168 may include comparators andother logic circuitry for determining if a reference emitted lightintensity signal is within a target range. If not within the desiredrange, the drive signal is adjusted by sensor control 168, e.g., in aniterative manner, until the target range is reached.

In an alternative embodiment, the reference emitted light intensitysignal provided by circuitry 164 is received by monitoring module 170.Monitoring module 170 may use the emitted light intensity and a detectedlight intensity to compute light attenuation at each wavelength. Theattenuation at each wavelength is used to compute second derivativeattenuation spectra as will be described in greater detail below forcomputing O₂Sat and HbT.

Alternatively, monitoring module 170 uses changes in the emitted lightintensity to adjust a computed absolute O₂Sat value. Absolute tissueO₂Sat may be computed assuming a constant emitted light intensity. Themeasured emitted light intensity may be used to adjust a computed O₂Sat.For example, an initially measured emitted signal intensity and acurrently measured emitted signal intensity (for one or all lightsources) can be used to adjust or correct an absolute tissue O₂Satcomputed using only the detected light signal and calibration constants.

As described above in conjunction with FIG. 5, a heating element 148 maybe included in sensor 180, or more generally in or coupled to device150, to operate in conjunction with sensor 180 for tissue oxygenationmonitoring. Heating element 148 is coupled to sensor input circuitry 162which provides operating signals, under the control of sensor control168, to heating element 148 to cause controlled heating of tissueadjacent to sensor 180.

A temperature sensor 146 may also be included in device 150 to allowlocal temperature monitoring in the vicinity of sensor 180. Temperaturesensor 146 is coupled to sensor output circuitry 166 to provide atemperature signal to monitoring module 170. The influence of changes inlocal tissue temperature may then be taken into account in monitoringtissue oxygenation measurements.

FIG. 7 is a flow chart 200 of one method for operating an optical sensorfor monitoring tissue oxygenation in an implantable or wearable medicaldevice. Flow chart 200 is intended to illustrate the functionaloperation of the device, and should not be construed as reflective of aspecific form of software or hardware necessary to practice the methodsdescribed. It is believed that the particular form of software, hardwareand/or firmware will be determined primarily by the particular systemarchitecture employed in the device and by the particular detection andtherapy delivery methodologies employed by the device. Providingsoftware to accomplish the described functionality in the context of anymodern medical device, given the disclosure herein, is within theabilities of one of skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

At block 202, a measurement time window is initiated. In variousapplications, tissue oxygenation monitoring may be continuous, periodic,or triggered in response to other events monitored by the medicaldevice. In the example shown in method 200, the monitoring is performedduring a periodic or triggered measurement window. After initiating themeasurement window, light emission is started at block 204. Lightemission at selected wavelengths may be controlled in a simultaneous,time multiplexed or frequency multiplexed manner or provided as pulsedor continuous white light.

At block 206, the electrical output signal generated by thephotodetector is measured. The output signal may be analyzed using anamplitude approach or an integration approach. In the integrationapproach, an integrator is included in the sensor output circuitry forintegrating the photodetector signal, for example using a capacitor. Thesignal may be integrated over fixed time intervals, which may be on theorder of 0.10 to 100 ms for example. The magnitude of the integratedsignal at the end of the fixed time interval is stored as a sample datapoint and corresponds to scattered light received by the light detectingportion of the optical sensor during the fixed time interval.

Alternatively, the photodetector signal may be integrated until apredetermined integrated signal magnitude is reached and the timeinterval required to reach the predetermined magnitude is stored as asample data point. When the integration approach is used to obtainsample data points, the fixed integration time interval or thepredetermined integrated signal magnitude may be selected to allow thesignal values to be acquired at or above the frequency of aphysiological condition of interest.

In other embodiments, the amplitude of the photodetector signal may beanalyzed directly by sampling the signal throughout the measurementwindow. Such sampling may correspond to sequential time intervals oflight source activation times during time multiplexed light sourceoperation. Alternatively the frequency may be selected to be greaterthan the greatest frequency modulation of a light source in the emittingportion to allow sampling all wavelengths of emitted light in afrequency multiplexed algorithm.

The measurement window may be set to allow time to acquire a desirednumber of output signal sample points for each of the desiredwavelengths. The photodetector signal amplitude or integrated signalamplitude or time interval continues to be sampled during themeasurement window until it expires as determined at decision step 208.

After acquiring the desired number of samples, the drive signalscontrolling the light emitting portion may be turned off and the sampleddata points may be stored and processed for computing O₂ Sat and HbT aswill be described further below. The sampled data points may be filteredor averaged at block 214 to provide smoothing of signal data or removalof artifact.

At blocks 210 and 212 corrections of sample data may be made to reducethe influence of ambient light and baseline offset. Correctionsperformed in blocks 210 and 212 may be executed before or afterfiltering at block 214. Ambient light may be measured directly bymeasuring the optical signal when the light emitting portion of theoptical sensor is not emitting light. The ambient light contribution maythen be subtracted from the light signal. Baseline offset (sometimesreferred to as the “dark signal” or “dark interval”) is caused bycurrent leakage within the optical sensor electronics that occurs in theabsence of light. Correction for the baseline offset for a given sensorcan be made based on a dark signal or dark interval for that sensor,measured, for example, at the time of device manufacture andqualification testing. If the baseline offset exceeds a desiredthreshold, offset correction may be included at block 212 to subtractthe offset from the incoming signal data. Corrections made at block 210and 212 may correspond to light signal corrections generally describedin U.S. patent application Ser. No. 12/039,242, hereby incorporatedherein by reference in its entirety.

FIG. 8 is a flow chart of a method for using an implantable or wearablemedical device including an optical sensor. At block 252 of method 250,the optical sensor is calibrated using control samples, for example inan in vitro blood circuit, having known oxygen saturation and totalhemoglobin concentration. The calibration method may be used to generatea look-up table. A look-up table of values relating light measurementscomputed from the photodetector output signal and the known O₂Sat andHbT may be stored in the device memory. The look-up table can then beused to derive absolute O₂Sat and Hbt values from an optical sensormeasurement.

Alternatively, calibration methods may include curve-fitting methods tosolve for coefficients defining best-fit curves to the calibration data.In one embodiment, the absolute tissue O₂Sat is defined by:

O₂Sat=Ae ^(B(SD″(λ))) +C   [1]

wherein SD″ is a scaled second derivative of the attenuation spectra ata selected wavelength λ. As will be further described below, a scaledsecond derivative of the attenuation spectra at a selected wavelength isdetermined by the monitoring module using the photodetector signal. Thescaled second derivative is the ratio of the second derivative withrespect to wavelength of the attenuation spectra at the selectedwavelength λ to the second derivative of the attenuation spectra atanother wavelength. By properly selecting the wavelength λ and the otherwavelength used for scaling, the scaled second derivative is anoxygen-dependent and measurement volume-independent ratio. Thecoefficients A, B and C are determined through best-fit analysis ofmeasurements of the scaled second derivative for calibration sampleshaving known oxygen saturation.

As used herein, a “volume-independent” measure of oxygen saturationrefers to a measurement that is substantially independent of the size ofthe optical sensor path that encompasses a measurement volume within asubstantially uniform tissue. In other words, in a uniform, homogenoustissue, a longer optical pathway that encompasses a larger measurementvolume and a relatively shorter optical pathway that encompasses asmaller measurement volume within the same uniform tissue will producesubstantially equal O₂Sat measurements. A volume-dependent measure ofoxygen saturation would be dependent on oxygen and the measurementvolume and would thus produce two different measurements for twodifferent measurement volumes in the same uniform, homogenous tissue.The second derivative method for computing O₂Sat as described hereineliminates scattering effects of a changing measurement volume andprovides a volume-independent measurement of O₂Sat.

A homogenous tissue is a tissue that includes structures that arerelatively small compared to the measurement volume. For example, ifmeasurement volume is related to emitting-to-detecting spacing, ahomogenous tissue might be a tissue wherein tissue structures orfeatures have a dimension of approximately 1/10 of theemitting-to-detecting spacing or less. A uniform tissue is a tissue thathas uniform oxygenation through the depth of the measurement volume incontrast to an oxygenation gradient. If a tissue is non-uniform ornon-homogeneous, different oxygen saturation measurements will beobtained depending on the optical path of the sensor through the tissue.

The total tissue hemoglobin volume fraction can be defined by theequation:

HbT=[M(100−O₂Sat)^(N) +L]*[(D″(A)_(λi) /dλ)/SF]  [2]

wherein M, N, and L are coefficients determined during calibration andD″(A)_(λi)/dλ is the second derivative of the attenuation spectra withrespect to wavelength at a selected intermediate wavelength λi. Thesecond derivative of the attenuation spectra with respect to wavelengthat a given wavelength is also referred to herein as D″(λ). D″(λ) ismeasured for samples containing known total hemoglobin volume fractionand known oxygen saturation. The calibration coefficients M, N and L maythen be computed for a best-fit of the measured second derivative valuesand known O₂ Sat and HbT. Alternatively, the measured second derivativevalues and known O₂Sat and HbT may be used to generate a look-up tablefor converting the measured second derivative values to HbT and O₂Satvalues.

SF is a spacing factor which may be used to adjust for anemitting-to-detecting portion spacing that may be different duringpatient monitoring than the spacing used during calibration. Since theHbT measurement is dependent on both oxygen and measurement volume, andmeasurement volume is dependent on the optical pathway defined at leastin part by the spacing between the emitting and detection portions, theHbT measurement needs to be corrected for changes inemitting-to-detecting portion spacing. For example, the sensor may becalibrated using a nominal emitting portion to detecting portionspacing, however when emitting and/or detecting portions are selectablein a sensor or combination of sensors, the spacing used during patientmonitoring may be different than that used during calibration. As such,a spacing factor corresponding to selectable emitting and detectingportion spacings may be stored and used to correct the HbT measurementwhen a different spacing is used during monitoring than duringcalibration.

At block 254, the sensor is positioned at a desired implant site (orexternal site in the case of an external device to be worn by thepatient). A test measurement is performed at block 256. The absoluteO₂Sat and HbT are determined from the sensor output signal using thestored calibration data. The measured values are compared to anacceptable measurement range at block 258. This comparison may beperformed manually or automatically using a programmed range stored inthe medical device memory.

If the O₂Sat exceeds a predefined expected range, for example greaterthan approximately 90%, the sensor may be in a position resulting inarterial blood strongly contributing to the O₂Sat measurement. If themonitoring application is concerned with measuring tissue oxygenationrather than arterial oxygen saturation, the sensor may be repositionedat block 264.

Likewise, if the O₂Sat is too low, for example less than approximately80%, the sensor may be in a position resulting in venous blood stronglycontributing to the O₂Sat measurement. If the absolute O₂Sat falls belowan expected physiological range for the particular sensing application,the sensor may be repositioned at block 264.

If the HbT is less than a predetermined range, for example less thanapproximately 1%, the sensor may be improperly positioned against thetissue (poor tissue contact) or positioned over a non-tissue medium orlow or non-perfused tissue. For example, if the sensor is positionedover fat, scar tissue, clear body fluids, or other implanted medicaldevice components, HbT may be below a normal physiological range forperfused tissue. HbT greater than an acceptable physiological range, forexample greater than approximately 25% for blood-perfused tissue, mayindicate blood pooling in the measurement volume beneath the sensor orother sensor measurement error. If the HbT test measurement is outside apredefined acceptable range, the sensor may be repositioned at block264.

Once the O₂Sat and HbT measurements are confirmed to be in an acceptablephysiological range at block 258, a tissue uniformity index may bedetermined at block 260. A tissue uniformity index is determined byutilizing at least two different emitting-to-detecting portion spacings.Accordingly at least two different combinations of light emitting andlight detecting portions at two different spacings must be available, onthe same or different optical sensors, positioned adjacent a targettissue volume. When at least two different spacings are available, theO₂Sat is measured using the two different spacings and compared. Atissue uniformity index may be computed based on the difference betweenthese two measurements, which would involve two different measurementvolumes defined by two different optical pathways. For example, arelatively greater emitting-to-detecting portion spacing would result ingreater depth of the optical pathway.

If the difference between the two measurements is small, the tissue isrelatively uniform and homogenous. If the difference between the twomeasurements is large, the tissue is non-uniform and/or heterogeneous. Athreshold for detecting uniform/homogenous versusnon-uniform/heterogeneous tissue volumes may be selected according to aparticular application. Detection of heterogenous tissue may warrantrepositioning of the sensor. A tissue uniformity index may indicate themost appropriate emitting-to-detecting spacing for measuring within adesired tissue volume. The initial O₂Sat, HbT and tissue uniformitymeasurements can therefore be used to decide if the sensor position isacceptable at block 262. If not, the sensor may be repositioned at block264.

If acceptable, the sensor is fixed at the desired site, and baselineO₂Sat and HbT measurements may be acquired and stored at block 266according to the needs of the particular monitoring application.Baseline measurements may be acquired for comparison to futuremeasurements, for use in learning algorithms performed during clinicalinterventions or naturally occurring pathological events, for use insetting thresholds for detecting physiological events, or for initiatingcontinuous monitoring of O₂Sat and HbT, i.e. tissue oxygenation.

FIG. 9 is a flow chart of a method 300 for operating an optical sensorfor monitoring tissue oxygenation. At block 302, the light emittingportion of the sensor is controlled to emit light by applying drivesignals to light sources. As described previously, light sources may becontrolled to emit light at different wavelengths in a sequential,time-multiplexed manner, in a simultaneous frequency-multiplexed manner,or simultaneously at multiple wavelengths when filtered in the detectingportion. Light emission may also be provided by a white light source.

A reference photodetector included in the light emitting portionprovides an output signal for measuring the intensity of light emittedby the sensor at block 308. The reference photodetector signal isdemodulated or otherwise processed to provide an intensity of lightemitted for each of the selected wavelengths at which attenuation willbe measured.

At block 304, the emitted light scattered by the tissue volume isdetected by the photodetector in the light detecting portion. Thedetecting portion provides an output signal corresponding to theintensity of light received. The output signal is demodulated orotherwise processed to provide an intensity of light received for eachof the selected wavelengths.

At block 306, the attenuation spectra is measured. In one embodiment,the attenuation of four wavelengths in the red to infrared spectrum ismeasured. The attenuation of the four different wavelengths may bemeasured using sequential detection of the different wavelengths by thephotodetector when a time multiplexed light emission control algorithmis used. Alternatively, measurement of the four different wavelengthsmay involve demodulation or filtering of simultaneously detected lightat the four different wavelengths when a frequency multiplexed orsimultaneous light emission control algorithm is used. In otherembodiments, remitted light from a white light source may be filtered toobtain the four different wavelength attenuation signals. In still otherembodiments, light sources configured for narrow-band light detectionmay be used to detect the four separate wavelengths. The attenuation (A)for a given wavelength (λ) can be measured as the negative logarithm ofthe ratio of the emitted light intensity (i_(in)) to the remitted lightintensity (i_(out)):

A(λ)=−log(i _(in) /i _(out))_(λ)  [3]

wherein i_(in) can be measured using a reference photodetector in thelight emitting portion of the sensor and i_(out) is measured using theoutput signal of the light detecting portion for a given wavelength. Theterm “attenuation” measurement as used herein generally refers to ameasure of the attenuation of light due to absorption and scattering bytissue along the optical path of the sensor. The measured attenuationmay therefore not be an exact measurement of the actual light absorptionby the tissue volume since light reflections and scattering can causeattenuation of the remitted light intensity not attributed to actuallight absorption by the tissue.

In some embodiments, the emitted intensity i_(in) for each wavelength ismeasured prior to implantation, e.g., at the time of manufacture, andassumed to be sufficiently stable throughout the usable life of thesensor as to not cause significant measurement error. In this case, areference photodetector may be eliminated from the light emittingportion of the sensor and thereby reduce overall size and complexity ofthe sensor. One method for measuring the emitted intensity prior toimplantation uses the light detecting portion to measure the remittedlight when the sensor is positioned within a calibrated reflectivehousing. The construction of the emitting portion is designed tominimize or prevent drift in the emitted light intensity over time.Design considerations include minimizing the distance between the tissueand the photonic surfaces of the light sources.

In some embodiments, the output signal intensity for each wavelength maybe normalized by a selected output signal intensity. Such normalizationwill cancel for uniform gain of the output signal. Uniform gain (whichmay be positive or negative) may result from known or unknown factors,such as motion resulting in a different optical sensor measurementvolume within a heterogenous tissue, drift in a light detectoramplifier, or partial blockage of the light detector window.

The attenuation for at least four wavelengths is measured to allow thesecond derivative with respect to wavelength of the attenuation spectrato be determined at two intermediate wavelengths. This determination ofsecond derivatives at two intermediate wavelengths allows forcomputation of a scaled second derivative. At block 310, the attenuationmeasurements for each the four detected wavelengths are used to computea second derivative (D″(λ)) of an intermediate wavelength λ, expressedgenerally as:

$\begin{matrix}{{D^{''}(\lambda)} = \frac{\begin{matrix}{\left\{ {\left( {{A\left( \lambda_{i + 1} \right)} - {A\left( \lambda_{i} \right)}} \right)/\left( {\lambda_{i + 1} - \lambda_{i}} \right)} \right\} -} \\\left\{ {\left( {{A\left( \lambda_{i} \right)} - {A\left( \lambda_{i - 1} \right)}} \right)/\left( {\lambda_{i} - \lambda_{i - 1}} \right)} \right\}\end{matrix}}{\left\lbrack {\left( {\lambda_{i + 1} - \lambda_{i - 1}} \right)/2} \right\rbrack}} & (4)\end{matrix}$

wherein A(λ_(i)) is the light attenuation, measured according toEquation 3 above, at the wavelength for which the second derivative isbeing computed, A(λ_(i+1)) is the attenuation at the next higherwavelength and A(λ_(i−1)) is the attenuation at the next lowerwavelength of the four wavelengths.

When the wavelength spacing between λ_(i) and neighboring wavelengths isequal, the denominators in the above Equation 4 may be dropped resultingin a simplified equation:

D″(λ_(i))=A(λ_(i+1))−2A(λ_(i))+A(λ_(i−1))   (5)

The second derivative of a selected intermediate wavelength is scaled bythe other computed second derivative at block 312 by computing a ratioof the two second derivatives. In one embodiment, the attenuation ismeasured for wavelengths at 680 nm, 720 nm, 760 nm, and 800 nm. Thesecond derivatives of the attenuation spectra are computed at 720 nm and760 nm and the second derivative at 720 nm is scaled by the secondderivative at 760 nm. The scaled second derivative (SD″) of the 720 nmattenuation can be expressed as:

SD″=D″(720)/D″(760)  (6)

This SD″(720) has been found to be dependent on oxygen saturation of thehemoglobin present in the measurement volume but independent of the sizeof the measurement volume, defined by the optical path of the sensor.Thus, SD″(720) is independent of the total hemoglobin present in themeasurement volume and independent of the optical path length. Thereduced dependence on total hemoglobin and optical path length isexpected to reduce the effects of motion artifact on a measurement ofO₂Sat based on SD″(720). Measuring attenuation for at least fourwavelengths allows the second derivatives of two intermediatewavelengths to be computed, allowing computation of a measurementvolume-independent, scaled second derivative.

Once the scaled second derivative is obtained, the stored calibrationdata is used at block 314 to derive the absolute tissue O₂Sat. Thesecond derivative for attenuation at 720 nm wavelength (and 760 nm) isdependent on oxygen saturation and total hemoglobin. Thus, at block 316,HbT may be determined knowing the second derivative of attenuation withrespect to wavelength at 720 nm, the derived absolute O2 Sat, and thestored calibration data.

If unequal wavelength spacings are used, a first derivative determinedat the numerical center of the unequally spaced wavelengths is firstdetermined. For example, if the wavelengths of 660, 720, 760, and 810were used, the first derivatives of the attenuation spectra atintermediate wavelengths corresponding to the numerical center of eachof these wavelength spacings would be expressed as:

$\frac{690}{\lambda} = {\frac{{A\; 720} - {A\; 660}}{720 - 660} = \frac{{A\; 720} - {A\; 660}}{60}}$$\frac{740}{\lambda} = {\frac{{A\; 760} - {A\; 720}}{760 - 720} = \frac{{A\; 760} - {A\; 720}}{40}}$$\frac{785}{\lambda} = {\frac{{A\; 810} - {A\; 760}}{810 - 760} = \frac{{A\; 810} - {A\; 760}}{50}}$

The second derivatives of two intermediate wavelengths occurring at thenumerical center between the first derivatives would be computed as:

$\frac{D\; 715}{\lambda^{2}} = {\frac{\frac{740}{\lambda} - \frac{690}{\lambda}}{740 - 690} = {\frac{{A\; 760} - {A\; 720}}{2000} - \frac{{A\; 720} - {A\; 660}}{3000}}}$$\frac{D\; 762.5}{\lambda^{2}} = {\frac{\frac{785}{\lambda} - \frac{740}{\lambda}}{785 - 740} = {\frac{{A\; 810} - {A\; 760}}{2250} - \frac{{A\; 760} - {A\; 720}}{1800}}}$

Based on an assumption of linear scattering, the two second derivativesmay be used to compute a scaled second derivative, e.g.D″(715)/D″(762.5) as a volume-independent measure of O₂Sat, which canthen be used with one of the second derivatives and the calibration datato compute a measure of HbT.

The illustrative equations above which compute the first and secondderivatives of the attenuation spectra at the numerical centers ofunequal wavelength spacings, and retain the denominators associated withwavelength spacing differences, may be adapted for use with any fourselected wavelengths. Selection of different wavelengths will change thecalibration constants, and may change the sensitivity of themeasurements to O₂Sat and HbT and the dependence of the measurements onthe tissue measurement volume as defined by the optical pathway of thesensor. Through careful selection of the emitted wavelengths for whichattenuation is measured, relatively greater sensitivity to O₂Sat andreduced dependence on measurement volume may be obtained.

Oxygen availability, as defined herein, is a function of both oxygensaturation of the hemoglobin present in the measurement volume and thetotal hemoglobin volume fraction. Depending on the particular monitoringapplication, the derived tissue O₂Sat and HbT may each be usedseparately in a monitoring algorithm or combined to determine a tissueoxygenation index (TOI) used to monitor a patient's status and/or detecta physiological condition. At block 322, a tissue oxygenation index maybe computed as a function of the absolute tissue oxygen saturation andthe total hemoglobin volume fraction. For example, a tissue oxygenationindex may be a weighted combination of the O₂Sat and HbT measurements.Thus, a tissue oxygenation index computed using absolute measurements ofO₂Sat and HbT can be available on a continuous or periodic basis in anambulatory patient.

The O₂Sat derived from a scaled second derivative is avolume-independent measurement and is therefore expected to have areduced susceptibility to motion artifact, which could alter the opticalpathway and thus alter the measurement volume. However, some embodimentsmay utilize the measured HbT, which is dependent on the measurementvolume, to filter or blank tissue oxygenation monitoring during periodsin which HbT is out of a normal range, which may be due to motion oractivity of the patient.

Accordingly, in one embodiment, the measured HbT is compared to anacceptable range, e.g. between approximately 1% and 25%, at block 318.If HbT is out of the acceptable range, tissue motion may be causingerroneous HbT measurements. At block 320, the tissue oxygenationmeasurement is blanked or otherwise deemed invalid based on theout-of-range HbT measurement. For example, patient activity may resultin oscillatory movements that produce a signal that is intermittently inand out of the acceptable range. Intervals in which the HbT measurementis out-of-range may be blanked for determining a tissue oxygenationindex. During intervals in which the HbT measurement is in range, thetissue oxygenation index is computed at block 322. When HbT is out ofrange, the absolute tissue oxygen saturation measurement may also beignored or still be determined and stored since it is avolume-independent measurement.

Alternatively, O₂Sat and HbT measurements may be filtered based on thefluctuation of HbT. If HbT variability is low, than a low rate ofaveraging O₂Sat and HbT measurements may be used. If HbT variabilityincreases, an increasing filtering or averaging frequency may be usedbased on the increased HbT variability.

FIG. 10 is a flow chart of an alternative method 400 for using anoptical sensor capable of measuring O₂Sat for monitoring tissueoxygenation. At block 402, control signals are applied to drivecircuitry to control the emission of light from the light emittingportion of the optical sensor.

In one embodiment, a reference photodetector is included in the lightemitting portion to provide a reference signal measuring the emittedlight. The intensity of the emitted light may be controlled using areference feedback signal as indicated by block 404. In the previousmethod 300 described in conjunction with FIG. 9, a referencephotodetector is used to measure the emitted light intensity forcomputing the attenuation of each wavelength using Equation 3 above. Inmethod 400, the emitted light intensity is measured using the referencephotodetector for controlling light emission such that the emittedintensity (i_(in)) at each of the wavelengths used for attenuationmeasurements is maintained within a specified range.

An emitted light reference signal measured at block 404 using thereference photodetector output signal is provided as a feedback signalto the control module controlling light emission at block 402. Drivesignals applied to the light emitting portion may be adjusted inresponse to the emitted light reference signal to maintain the emittedlight intensity within a target range for each wavelength selected forattenuation measurements.

When the emitted light is controlled to be maintained within a specifiedrange, the emitted light intensity (i_(in)) in the attenuation equation(3) above becomes a constant. Manipulation of the second derivativeequation (5) above results in a modified second derivative equation:

D″(λ_(i))_(modified) =C _(i)−log(i _(out))_(λi+1)+2log(i_(out))_(λi)−log(i _(out))_(λi−1)   (7)

which may be rewritten as:

D″(λ_(i))_(modified) =C _(i)+log {(i _(out))_(λi) ²/(i _(out)_(λi+1))(_(out))_(λi−1))}  (8)

The term C_(i) for a given wavelength λ_(i) becomes a calibrationconstant.

Thus, a modified scaled second derivative may be computed using only thedetecting portion output signal (and calibration constants C_(i)determined for each of the measured wavelengths) without using ameasurement of emitted light intensity for computing attenuation of theemitted light. In the case where there is no reference measurement foremitted light intensities at each wavelength, but the drive signal tothe light sources is controllable, the constants Ci are predeterminedfunctions of the light source drive signal. Note that the above Equation8 is written for equal wavelength spacings and will include more termsfor non-equal wavelength spacings (see Equation 4 above) in which casethe denominators in the derivative equations corresponding to wavelengthspacing differences need be retained.

The scattered light is detected by the optical sensor at block 406 andused to compute the modified second derivatives at block 408 at two (ormore) intermediate wavelengths. The modified second derivatives needonly be computed for the two wavelengths being used to compute O₂Sat andHbT.

A simplified scaled second derivative may be used as an estimate ofO₂Sat in which the C_(i) constants are ignored in the above equations. Asimplified scaled second derivative may take the form of:

$\begin{matrix}{{SD}^{''} = \frac{{- {\log \left( i_{out} \right)}_{{\lambda \; i} + 1}} + {2\; {\log \left( i_{out} \right)}_{\lambda \; i}} - {\log \left( i_{out} \right)}_{{\lambda \; i} - 1}}{{- {\log \left( i_{out} \right)}_{{\lambda \; i} + 2}} + {2{\log \left( i_{out} \right)}_{{\lambda \; i} + 1}} - {\log \left( i_{out} \right)}_{\lambda \; i}}} & (9)\end{matrix}$

This simplified scaled second derivative may be useful for measuring anon-calibrated, index of tissue oxygen saturation at block 410. Acorresponding non-calibrated index of HbT may be computed at block 412using the simplified second derivative computed using equation 9. TheO₂Sat and HbT indices may be used individually or combined in a tissueoxygenation index computed as a function of both at block 414.

In addition or alternatively to using the emitted light reference signalas feedback to control light emission, the emitted light referencesignal may be used by the monitoring module to adjust the computedmodified second derivatives at block 408. Shifts in the intensity of theemitted light may be accounted for by introducing a correction term (CT)in the equation used to compute the modified second derivative.Accordingly, an adjusted modified second derivative for a selectedintermediate wavelength used to compute absolute oxygen saturation mightbe computed using:

D″(λ_(i))_(modified) =C _(i)−log(i _(out) +CT)_(λi+1)+2log(_(out)+CT)_(λi)−log(_(out) +CT)_(λi−1)   (9)

wherein CT is a correction term determined for each wavelength using theemitted light reference signal and is used to adjust the remitted lightintensities i_(out) for each wavelength.

FIG. 11 is a flowchart of a method of monitoring tissue oxygenation in amedical device system. It is understood that the medical device systemdescribed in FIG. 11 may correspond to a system associated with thesensor alone, or may correspond to the sensor being incorporated in animplantable medical device, as shown in FIG. 1D. As described above, itis desirable that the optical sensor be positioned over substantiallyhomogenous tissue, and that obtaining measurements at different tissuesdepths can be used to provide a measure of tissue uniformity. Therefore,according to one embodiment, in order to obtain measurements atdifferent tissue depths and volumes, the functionality of the modularassemblies 102 and 102′ of the optical sensor is chosen to define afirst optical pathway 110 and a second optical pathway 112, describedabove. An O₂Sat measurement is determined along the first opticalpathway 110, block 500, and an O₂Sat measurement is determined along thesecond optical pathway 112, block 502. The two O₂Sat measurements arecompared, Block 504, and a determination is made as to whether adifference between the O₂Sat measurements is greater than a uniformitythreshold, block 506. According to one embodiment, for example, theuniformity threshold is set as a percentage, such as a 2 percentdifference. In another embodiment, the uniformity threshold may includemultiple thresholds to identify levels of uniformity, such as a firstthreshold for identifying the tissue as essentially homogenous and asecond threshold for identifying the tissue as being nearly homogenous.For example, the first threshold may be set at 2 percent and the secondthreshold may be set as 5 percent, so that the tissue is identified asbeing essentially homogenous if it is determined that the differencebetween the O₂Sat measurements is less than 2 percent, and as beingnearly homogenous if it is determined that the difference between theO₂Sat measurements is not less than 2 percent, but less than 5 percent.

If the difference between the two O₂Sat measurements measured along thetwo optical pathways 110 and 112 is not greater than the uniformitythreshold, the tissue over which the optical sensor is positioned isdetermined to be homogenous, or uniform, block 508, and the oxygensaturation signal derived from the optical sensor is utilized by thedevice to confirm detection of a cardiac event. If the differencebetween the two O₂Sat measurements is greater than the uniformitythreshold, the tissue over which the optical sensor is positioned isdetermined to be non-uniform, block 510, and the functionality of theoptical sensor is changed, block 512. For example, changing thefunctionality of the optical sensor may correspond to the optical signalbeing no longer utilized in the detection process, or in the opticalpathways associated with the modular assemblies 102 and 102′ beingreselected, or reduced to just a single pathway. According to anotherembodiment, if the tissue is determined to be non-uniform, adetermination made be made as to whether the two O₂Sat measurementsbehave similarly

FIG. 12 is a top schematic view of a sensor according to anotherembodiment. As illustrated in FIG. 12, an optical sensor 600 may includefour modular assemblies 602, 602′, 620, and 620′. Specifically, sensor600 includes a first portion 601 having assemblies 602 and 602′ includedtherein, with each assembly including light sources 606 and 606′ andlight detectors 608 and 608′, respectively, and a second portion 603having assemblies 620 and 620′ included therein, with each assemblyincluding light sources 622 and 622′ and light detectors 624 and 624′,respectively. Similar to sensor 100 described above, the functionalityof each assembly 602, 602′, 620, and 620′ is selectable such that oneassembly is selected to operate as an emitting portion and the otherassembly is selected to operate as a detecting portion. This functionalselection may be made at the time of manufacture and not alterablethereafter. Alternatively, this selection may be dynamic under thecontrol of control circuitry included in the associated medical device.Functional selection of the assemblies may be based on user input or inresponse to feedback or self-diagnostic measurements made by sensor 600.

By arranging the light sources and the light detectors in a particularspatial manner with respect to one another, four different opticalpathways 610, 612, 626 and 630, and thus four different measurementvolumes, may be realized depending on the selection of the functionalityof each assembly. If assembly 602 is selected as the detecting portionand assembly 602′ is selected as the emitting portion, theemitting-to-detecting spacing 604 is relatively shorter than a spacing605 that would result if functional selection of assemblies 602 and 602′were reversed (i.e., assembly 602 selected as emitting and assembly 602′selected as detecting). Similarly, if assembly 620 is selected as thedetecting portion and assembly 620′ is selected as the emitting portion,the emitting-to-detecting spacing 604 is relatively shorter than aspacing 605 that would result if functional selection of assemblies 620and 620′ were reversed (i.e., assembly 620 selected as emitting andassembly 620′ selected as detecting). The resulting longeremitting-to-detecting spacing 605 will result in relatively longer (anddeeper) optical pathways 612 and 630 (shown schematically), whenassemblies 602 and 620 are emitting light, than the optical pathways 610and 626 (shown schematically) that results when assemblies 602′ and 620′are emitting light.

As such, through proper orientation of assemblies 602, 602′, 620, and620′ and the emitting and detecting components therein, differentoptical pathways may be selected through the selectable functionality ofthe assemblies 602, 602′, 620, and 620′, including variable spacing forvariable tissue depths.

FIG. 13 is a flowchart of a method of monitoring tissue oxygenation in amedical device system. According to an embodiment, the use of multiplesensor assemblies enables the device to better account for the effectson the optical sensor that may occur as a result of undesired movementof the sensor, or due to motion or posture changes of the patient.Therefore, according to one embodiment, in order to account for motioneffects on the sensor, the functionality of the modular assemblies 602,602′, 620, and 620′ of the optical sensor is chosen to define respectivefirst optical pathways 610 and 626 and second optical pathways 612 and630, described above.

In the example illustrated in FIG. 12, the first optical pathways 610and 626 have approximately equal emitting-to-detecting spacing 604, andare laterally spaced a distance 609 from each other. Similarly, thesecond optical pathways 612 and 630 have approximately equalemitting-to-detecting spacing 605, and are also laterally spaceddistance 609 from each other. In this way, sensor 600 is able todetermine variations in O₂Sat measurements along two separateperpendicular pathways through tissue to detect variations in tissueuniformity extending both laterally in the direction of arrow 609, andlaterally in the direction of arrow 605, along sensor 600. Thismulti-dimensional sensing increases the ability of the device to betteraccount for effects on sensor 600 due to undesired motion or drift ofthe sensor 600, or due to motion or posture changes of the patient.

For example, if the uniformity of tissue between modules 602 and 602′changes by a significant amount, while the uniformity of tissue betweenmodules 620 and 620′ remains relatively constant, it is likely thateither shifting or drift of the sensor 600 or motion of the patient hasoccurred. A determination can then be made as to whether the sensor 600can continue to be utilized using the O₂Sat measurement from bothportions 601 and 603 of the sensor 600, using only the O₂Sat measurementfrom one of the two portions 601 and 603, i.e., portion 603 in thisexample, or if the sensor 600 should be disabled or re-positioned. Inaddition, an alert may be delivered to indicate possible drift of thesensor 600 to the patient or clinician.

For example, as illustrated in FIG. 13, in order to determine tissueuniformity, an O₂Sat measurement is determined for both the firstportion 601 and the second portion 603 of the sensor 600. In particular,an O₂Sat measurement is determined, block 700, along the first opticalpathway 610 of the first portion 601 and along the second opticalpathway 612 of the first portion 601 of sensor 600, block 702. The twoO₂Sat measurements are compared, Block 704, and a uniformity isidentified for the first portion 601 of the sensor 600, block 706.

For example, in order to identify the uniformity for the first portion601 of the sensor 600, a determination is made in block 706 as towhether a difference between the two O₂Sat measurements for the firstportion 601 is greater than a uniformity threshold. If the differencebetween the two O₂Sat measurements measured along the two opticalpathways 610 and 612 is not greater than the uniformity threshold, thetissue over which the first portion 601 of the optical sensor ispositioned is determined to be homogenous, or uniform in block 706. Ifthe difference between the two O₂Sat measurements is greater than theuniformity threshold, the tissue over which the first portion 601 of theoptical sensor is positioned is determined to be non-uniform in block706.

Similarly, an O₂Sat measurement is determined, block 708, along thefirst optical pathway 626, and along the second optical pathway 630 ofthe second portion 601 of the sensor 600, block 710. The two O₂Satmeasurements are compared, Block 712, and a uniformity is identified forthe second portion 603 of the sensor 600, block 714.

For example, in order to identify the uniformity for the second portion603 of the sensor 600, a determination is made in block 714 as towhether a difference between the two O₂Sat measurements for the secondportion 603 is greater than the uniformity threshold. If the differencebetween the two O₂Sat measurements measured along the two opticalpathways 626 and 630 is not greater than the uniformity threshold, thetissue over which the second portion 603 of the optical sensor ispositioned is determined to be homogenous, or uniform in block 714. Ifthe difference between the two O₂Sat measurements is greater than theuniformity threshold, the tissue over which the second portion 603 ofthe optical sensor is positioned is determined to be non-uniform inblock 714.

Once a uniformity has been identified for both the first portion 601 andthe second portion 603 of the sensor 600, a determination is made as towhether both portions 601 and 603 of the sensor 600 are positioned overuniform tissue, block 716. If both the first portion 601 and the secondportion 603 are identified as uniform, block 718, the oxygen saturationsignal derived from both portions 601 and 603 of the sensor 600 isutilized to confirm detection of a cardiac event by the device. If boththe first portion 601 and the second portion 603 are not identified asuniform, a determination is made as to whether one of the first portion601 and the second portion 603 is uniform, block 720.

If one of the first portion 601 and the second portion 603 is uniform,the functionality of the sensor 600 is changed, block 722, and thesensor 600 operates under the new functionality. For example, changingthe functionality may include disabling the sensor altogether, or mayinclude continuing to utilize the sensor 600 using only the portiondetermined to be positioned over uniform tissue. In addition, in eithercondition, an alert may be generated to indicate to the patient orclinician that a change in the uniformity of the tissue over which thesensor is positioned, or in the quality of the measurement has occurred.If one of the first portion 601 and the second portion 603 is notuniform, i.e., both portions 601 and 603 are positioned over non-uniformtissue, the sensor 600 is disabled, 724, and/or an alert may begenerated to indicate to the patient or clinician that a change in theuniformity of the tissue over which the sensor is positioned, or in thequality of the measurement has occurred.

Thus, a medical device and associated methods have been presented in theforegoing description with reference to specific embodiments. It isappreciated that various modifications to the referenced embodiments maybe made without departing from the scope of the disclosure as set forthin the following claims.

1. A medical device for detecting signals, comprising: a first opticalassembly to detect emitted light scattered by a volume of tissuedelivered along a first pathway to generate corresponding first detectedlight intensity output signals; a second optical assembly to detectemitted light scattered by the volume of tissue delivered along a secondpathway different from the first pathway to generate correspondingsecond detected light intensity output signals; and a processorconfigured to determine whether a difference between the emitted lightdetected along the first pathway and the emitted light detected alongthe second pathway is greater than a predetermined threshold, and toalter sensing by the device in response to the determining whether adifference is greater than the predetermined threshold.
 2. The device ofclaim 1, further comprising a monitoring module couple to the processorand configured to determine a first oxygen saturation measurementcorresponding to detected light emitted along the first pathway inresponse to the first detected light intensity output signals, and todetermine a second oxygen saturation measurement corresponding todetected light emitted along the second pathway in response to thesecond detected light intensity output signals, wherein the processor isfurther configured to compare the first and second oxygen saturationmeasurement and determine whether a difference between the first andsecond oxygen saturation measurement is greater than the predeterminedthreshold.
 3. The device of claim 2, wherein altering sensing by thedevice comprises selecting between the first pathway and the secondpathway.
 4. The device of claim 2, wherein the processor is furtherconfigured to determine levels of uniformity of the volume of tissue. 5.The device of claim 4, wherein the processor is further configured todetermine whether a difference between the emitted light detected alongthe first pathway and the emitted light detected along the secondpathway is less than a first level threshold, identify the volume oftissue as having a first tissue uniformity in response the differencebeing less than the first level threshold, determine whether adifference between the emitted light detected along the first pathwayand the emitted light detected along the second pathway is greater thana second level threshold different from the first level threshold, andidentify the volume of tissue as having a second tissue uniformity inresponse the difference being both less than the second level thresholdand not less than the first level threshold.
 6. The device of claim 5,wherein the processor is further configured to select between the firstpathway and the second pathway in response to the identifying.
 7. Thedevice claim 1, wherein the monitoring module is further configured todetermine an attenuation measurement for each wavelength of a pluralityof wavelengths of the first pathway and the second pathway in responseto the first and second detected light intensity output signals,determine a second derivative of the attenuation measurement for a firstwavelength and a second wavelength of a plurality of wavelengthscorresponding to the first pathway in response to only the firstdetected light intensity output signals, determine a second derivativeof the attenuation measurement for a first wavelength and a secondwavelength of a plurality of wavelengths corresponding to the secondpathway in response to only the second detected light intensity outputsignals, determine a first oxygen saturation measurement correspondingto detected light emitted along the first pathway in response to thedetermined second derivative associated with the first pathway, anddetermine a second oxygen saturation measurement corresponding todetected light emitted along the second pathway in response to thedetermined second derivative associated with the second pathway, andwherein the processor is further configured to compare the first andsecond oxygen saturation measurement.
 8. The device of claim 7, whereinthe processor is further configured to determine levels of uniformity ofthe volume of tissue.
 9. The device of claim 8, wherein the processor isfurther configured to determine whether a difference between the emittedlight detected along the first pathway and the emitted light detectedalong the second pathway is less than a first level threshold, identifythe volume of tissue as having a first tissue uniformity in response thedifference being less than the first level threshold, determine whethera difference between the emitted light detected along the first pathwayand the emitted light detected along the second pathway is greater thana second level threshold different from the first level threshold, andidentifying the volume of tissue as having a second tissue uniformity inresponse the difference being both less than the second level thresholdand not less than the first level threshold.
 10. The device of claim 9,wherein the processor is further configured to select between the firstpathway and the second pathway in response to the identifying.
 11. Thedevice of claim 1, wherein the monitoring module is further configuredto determine an attenuation measurement for each wavelength of aplurality of wavelengths of the first pathway and the second pathway inresponse to the first and second detected light intensity outputsignals, determine a second derivative of the attenuation measurementfor a first wavelength and a second wavelength of a plurality ofwavelengths corresponding to the first pathway in response to only thefirst detected light intensity output signals, determine a secondderivative of the attenuation measurement for a first wavelength and asecond wavelength of a plurality of wavelengths corresponding to thesecond pathway in response to only the second detected light intensityoutput signals, determine a scaled second derivative of the attenuationmeasurement of the first wavelength for the first pathway using thedetermined second derivative of the attenuation of the second wavelengthfor the first pathway, determine a first oxygen saturation measurementcorresponding to detected light emitted along the first pathway inresponse to the determined scaled second derivative for the firstpathway, determine a scaled second derivative of the attenuationmeasurement of the first wavelength for the second pathway using thedetermined second derivative of the attenuation of the second wavelengthfor the second pathway, and determine a second oxygen saturationmeasurement corresponding to detected light emitted along the firstpathway in response to the determined scaled second derivative for thesecond pathway, wherein the processor is further configured to comparethe first and second oxygen saturation measurement.
 12. A computerreadable medium having computer executable instructions for performing amethod comprising: detecting emitted light scattered by a volume oftissue delivered along a first pathway at a plurality of wavelengths togenerate corresponding first detected light intensity output signals;detecting emitted light scattered by the volume of tissue deliveredalong a second pathway different from the first pathway at a pluralityof wavelengths to generate corresponding second detected light intensityoutput signals; determining whether a difference between the emittedlight detected along the first pathway and the emitted light detectedalong the second pathway is greater than a predetermined threshold; andaltering sensing by the device in response to the determining whether adifference is greater than the predetermined threshold.